Silver-bismuth non-contact metallization pastes for silicon solar cells

ABSTRACT

Metallization pastes for use with semiconductor devices are disclosed. The pastes contain silver particles, low-melting-point base-metal particles, organic vehicle, and optional crystallizing agents. Specific formulations have been developed that produce stratified metal films that contain less silver than conventional pastes and that have high peel strengths. Such pastes can be used to make high contact resistance metallization layers on silicon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application62/209,885, filed Aug. 26, 2015 and to U.S. Provisional PatentApplication 62/377,369, filed Aug. 19, 2016, both of which areincorporated by reference herein.

STATEMENT OF GOVERNMENT SUPPORT

This invention is made with Government support under contract numberIIP-1430721 awarded by the NSF. The Government may have certain rightsin this invention.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to conductive metallization pastes, and morespecifically to metallization layers that have a high peel strengthafter co-firing and subsequent soldering to a tabbing ribbon. Suchpastes may be especially well-suited for use in silicon based solarcells.

In general, conductive metallization pastes contain silver particles,glass frit, and an organic binder, all mixed with an organic vehicle.The price of silver particles represents more than 80% of the materialscost of many conductive metallization pastes, especially those used forfront side and rear tabbing solar cell applications. Reducing the amountof silver in the paste is highly desirable and is done either byreducing the overall paste solids content or by replacing silver withother particles to reduce the silver fraction in the paste.

State-of-the-art commercial rear tabbing pastes contain organic solventsand binder along with 40-55 wt % silver and 5-8 wt % glass frit. Whenthe total solids content of the paste is reduced, the overall fired filmthickness decreases. This strategy has been successfully employed overthe last five years in the rear tabbing layer for PV (photovoltaic)cells, reducing the fired silver film thickness from 7 μm to 3 μm. Suchpastes result in films where the Ag content is 83-90 wt % of the driedfilm. Unfortunately, further reduction of the fired film thickness canlead to increased film porosity, incomplete Ag coverage on the siliconsurface, and solder leaching, all of which reduce the overall peelstrength of the tabbed solder joint.

In the past, adding a large quantity of non-precious metal, inorganicparticles to Ag based metallization pastes has proven challenging forseveral reasons. Adding large quantities of particles that containnon-precious metal, inorganic materials such as aluminum that alloy withsilver at high temperatures can form dense, strong films, but theresulting alloy may be unsolderable, resulting in a soldered joint withan unacceptable low (e.g., less than 1 N/mm) peel strength. Particlesmade from non-precious metal, inorganic materials such as nickel, thatare not miscible with silver may produce mixed Ag/inorganic films thatretain solderability at high firing temperatures. However, silver doesnot wet such materials during firing, which can cause de-sinteringaround the inorganic particles and result in phase-separated films thatmay be homogenous but may also be highly porous and structurally weak.Furthermore, adding large quantities of base-metal particles cannegatively impact the contact resistance between the tabbing layer andaluminum layer, causing an increase in the overall series resistance ofthe solar cell.

There is a need to develop metallization pastes that contain inorganicmaterials to reduce their silver content. It would be especially usefulif such pastes produced fired films with high peel strengths andlong-term joint reliability while maintaining similar or betterelectrical performance than films fade from conventional silvermetallization pastes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and others will be readily appreciated by theskilled artisan from the following description of illustrativeembodiments when read in conjunction with the accompanying drawings.

FIG. 1 is a schematic cross-section drawing of a stratified film 100made from co-firing metallization pastes on a silicon substrate,according to an embodiment of the invention.

FIG. 2 is a schematic drawing that shows the front (or illuminated) sideof a silicon solar cell, according to an embodiment of the invention.

FIG. 3 is a schematic drawing that shows the rear side of a siliconsolar cell, according to an embodiment of the invention.

FIG. 4 is a scanning electron microscope (SEM) image (10 k×magnification) of a stratified Ag:Bi film on a silicon wafer, accordingto an embodiment of the invention.

FIG. 5 is an x-ray diffraction (XRD) pattern from a stratified Ag:Bifilm on a silicon substrate, according to an embodiment of theinvention.

FIG. 6A is a cross-section schematic illustration that shows a portionof the rear side of a silicon solar cell before co-firing, according toan embodiment of the invention.

FIG. 6B is a cross-section schematic illustration that shows a portionof the rear side of the silicon solar cell shown in FIG. 6A afterco-firing, according to an embodiment of the invention.

FIG. 7 is a scanning electron microscope (SEM) image (3 k×magnification) of an overlap region on the silicon solar cell whereinthe rear tabbing layer does not contain aluminum-based particles.

FIG. 8 is a scanning electron microscope (SEM) image (3 k×magnification) of an overlap region on the silicon solar cell whereinthe rear tabbing layer contains aluminum based particles, according toan embodiment of the invention.

SUMMARY

In one embodiment of the invention, a solar cell is disclosed. The solarcell has a silicon substrate, a plurality of fine grid lines on thefront surface of the silicon substrate, at least one front busbar layerin electrical contact with at least a portion of the plurality of finegrid lines, an aluminum layer on a portion of the back surface of thesilicon substrate, and at least one rear tabbing layer on a portion ofthe back surface of the silicon substrate. In one arrangement, the reartabbing layer is a stratified film that has at least two sublayers: afirst low-melting-point base-metal sublayer over the silicon substrate,and a second silver-rich sublayer over the first sublayer. The secondsublayer may have an exterior surface that is exposed to the outsideenvironment. The stratified film has a low-temperature, base-metalfraction greater than 20%. Some edges of the aluminum layer may overlapwith some edges of the rear tabbing to form overlap regions. The overlapregions may contain a solid aluminum-silicon eutectic layer in theunderlying silicon layer, a modified rear tabbing layer over the solidaluminum-silicon eutectic layer, and an aluminum layer over the modifiedrear tabbing layer. The stratified film may also contain(MO_(x))_(y)(SiO₂)_(z), crystallites wherein 0≤x≤3, 1≤y≤10, and 0≤z≤1,and M is any of bismuth, tin, tellurium, antimony, lead, silicon, oralloys, composites, or combinations thereof.

The first sublayer may contain a material such as bismuth, aluminum,carbon, tin, tellurium, antimony, lead, silicon, or alloys, oxides,composites, or combinations thereof. The first low-melting-pointbase-metal sublayer may contain bismuth and oxygen.

The stratified film may contain at least 0.5 wt % aluminum. Thestratified film may have a low-temperature, base-metal fraction greaterthan 30%, greater than 40%, or greater than 50%.

The aluminum layer may have a thickness between 20 μm and 30 μm.

The stratified film may have a thickness between 1 μm and 15 μm, between1 μm and 10 μm, or between 1 μm and 3 μm. The first sublayer may have athickness between 0.01 μm and 5 μm. The first sublayer may have athickness between 0.25 μm and 3 μm, or between 0.5 μm and 2 μm. Thesolar cell of claim 1, wherein the second sublayer may have a thicknessbetween 0.5 μm and 10 μm, between 0.5 μm and 5 μm, or between 1 μm and 4μm.

The solar cell of claim 1, wherein the second sublayer exterior surfacemay contain at least 90 wt %, at least 80 wt %, or at least 70 wt %silver.

The aluminum layer in the overlap region may have an exterior surfacethat is exposed to the outside environment. The modified rear tabbinglayer in the overlap region may have an exterior surface that is exposedto the outside environment. The overlap region may be between 10 μm and500 μm or 100 μm and 300 μm wide.

The modified rear tabbing layer may contain any of bismuth, tin,tellurium, antimony, lead, silicon, oxides or alloys, composites, orcombinations thereof. The modified rear tabbing layer may contain atleast 0.5 wt % or at least 1 wt % aluminum.

The stratified film may have a conductivity that is 2 to 50 times, 2 to25 times, or 2 to 10 times less than the conductivity of bulk silver. T

The rear tabbing layers may have a peel strength of more than 1 N/mmwhen soldered to tin-coated copper tabbing ribbons.

The contact resistance between the rear tabbing layer and the aluminumlayer may be between 0 and 5 mΩ, between 0.25 and 3 mΩ, or between 0.3and 1 mΩ.

The rear surface of the silicon substrate may have a doped silicon baselayer, and the rear tabbing layers make electrical contact to thesilicon base layer with a contact resistance greater than 100 mΩ-cm2.

The silicon substrate may contain be a monocrystalline or amulti-crystalline silicon wafer and may have an n-type base or a p-typebase.

In another embodiment of the invention, a metallization paste isdisclosed. The metallization paste contains silver particleslow-melting-point, base-metal particles, optionally a crystallizingagent and aluminum-based particles, which are all mixed together in anorganic vehicle. The low-temperature, base-metal fraction in themetallization paste is greater than 20%, greater than 30%, greater than40%, or greater than 50%. The metallization paste may contain acrystallizing agent. The metallization paste may have a solids loadingbetween 30 wt % and 70 wt %. The metallization paste may contain between10 wt % and 45 wt % silver particles. The metallization paste maycontain between 5 wt % and 35 wt % low-melting-point, base-metalparticles particles. The metallization paste may contain between 0.5 wt% and 3 wt % aluminum particles. The metallization paste may alsocontain less than 2 wt % strong glass-forming frits.

The silver particles may be spherically shaped nanoparticles with a D50between 10 nm and 1 μm, between 50 nm and 800 nm, or between 200 nm and500 nm. The silver particles may be spherically shaped micron-sizedparticles with a D50 between 1 μm and 10 μm, between 1 μm and 5 μm, orbetween 1 μm and 2 μm. The silver particles may be flake shapedparticles with a width between 1 and 10 μm and a thickness between 100nm and 500 nm. The silver particles may be a mixture of nanoparticlesand micron sized particles. The silver particles may be a mix of 50 wt %nanoparticles with a D50 of 500 nm, 30 wt % spherical micron sizedparticles with a D50 of 2 μm, and 20 wt % silver flakes with diameter of2 μm. The silver particles may have either a unimodal or a bimodal sizedistribution.

The low-melting-point, base-metal particles may contain any of bismuth(Bi), tin (Sn), tellurium (Te), antimony (Sb), lead (Pb), or alloys,composites, or other combinations thereof. The low-melting-point,base-metal particles may contain bismuth. The metallizationlow-melting-point, base-metal particles may be crystalline. Thelow-melting-point, base-metal particles may have a spherical shape witha D50 between 50 nm and 5 μm, between 300 nm and 5 μm, or between 300 nmand 2 μm. The low-melting-point, base-metal particles may have either aunimodal or a bimodal size distribution.

The low-melting-point, base-metal particles may have bismuth cores andeach bismuth core is coated by a shell. The shell may contain any ofsilver (Ag), nickel (Ni), nickel-boron (Ni:B) alloy, tin (Sn), tellurium(Te), antimony (Sb), lead (Pb), molybdenum (Mo), titanium (Ti),magnesium (Mg), boron (B), silicon oxides (SiOx), composites, or othercombinations thereof. The shell may contain silver with a thickness isless than 200 nm. The shell may contain a nickel-boron alloy with aboron content between 2-8 wt %.

The aluminum-based particles may have a spherical shape with a D50between 50 nm and 15 μm, between 300 nm and 10 μm, or between 300 nm and4 μm. The aluminum-based particles may have either a unimodal or abimodal size distribution. The metallization paste may contain 35 wt %silver particles, 16 wt % low melting point base-metal particles, 1 wt %aluminum-based particles, 48 wt % organic vehicle, no crystallizingagent, and no glass frit.

The metallization paste may have a viscosity between 10,000 and 200,000cP at 25° C. and at a sheer rate of 4 sec-1.

The metallization paste may also contain a crystallizing agent mixedtogether with the silver particles, the low-melting-point base-metalparticles, and the aluminum-based particles in the organic vehicle. Themetallization paste may contain less than 1 wt % or less than 0.1 wt %crystallizing agent. The crystallizing agent may contain any oftellurium, silicon , boron, zinc, or oxides or alloys thereof. Thecrystallizing agent may contain tellurium and/or tellurium oxide. Thecrystallizing agent may contain particles with an approximatelyspherical shape and a D50 between 50 nm and 5 μm, between 100 nm and 3μm, or between 200 nm and 1 μm. The crystallizing agent may containflake particles with diameters between 1 and 10 μm and thicknessesbetween 100 nm and 500 nm.

In another embodiment of the invention, an overlap region on the backside of a silicon solar cell is disclosed. The overlap region contains asolid aluminum-silicon eutectic region that extends into the back sideof the silicon solar cell, a modified rear tabbing layer over the solidaluminum-silicon eutectic region, and an aluminum layer over themodified rear tabbing layer. The modified rear tabbing layer may containany of bismuth, tin, tellurium, antimony, lead, silicon, or alloys,composites, or combinations thereof. The low-melting-point base-metalfraction greater than 20%.

DETAILED DESCRIPTION

The preferred embodiments are illustrated in the context ofmetallization layers for silicon-based solar cells. The skilled artisanwill readily appreciate, however, that the materials and methodsdisclosed herein will have application in a number of other contextswhere making good electrical contact to semiconducting or conductingmaterials is desirable, particularly where good adhesion and low costare important. These and other objects and advantages of the presentinvention will become more fully apparent from the following descriptiontaken in conjunction with the accompanying drawings. All publicationsreferred to herein are incorporated by reference in their entirety forall purposes as if fully set forth herein.

Metallization pastes are formulated to be applied onto substrates, suchas solar cells, using a process such as screen printing. The pastes aredried and then co-fired in an oxidizing ambient to compact the solidcomponents and vaporize and oxidize organic molecules in order toproduce compact films. For the purposes of this disclosure, the term“co-fired” is used to describe heating to between 700° C. and 850° C.for about 0.5 to 3 seconds in ambient air conditions using an IR beltfurnace or similar tool. The temperature profile of the substrate isoften calibrated using a DataPaq® system with a thermocouple attached toa bare substrate.

Metrics for such films include:

-   -   solderability    -   peel strength,    -   bulk resistance,    -   contact resistance between the rear tabbing layer and the        aluminum layer, and    -   contact resistance between the metallization layer and the        substrate.

Solderability is the ability to form a strong physical bond between twometal surfaces by the flow of a molten metal solder between them attemperatures below 400° C. Soldering on a solar cell may be performedafter heating the cell in air to over 650° C. for approximately onesecond. Soldering involves the use of flux, which is a chemical agentthat cleans or etches one or both of the surfaces prior to reflow of themolten solder. This can be difficult because many metal oxides areresistant to commonly-used fluxes after oxidation above 650° C.

For layers that contact a tabbing ribbon directly, it is useful if thepeel strength is greater than 1N/mm (Newton per millimeter). The peelstrength can be described as the force required to peel a solderedribbon, at a 180° angle from the soldering direction, divided by thewidth of the soldered ribbon. It is common for the peel strength betweenrear tabbing layers and the solar cell to be between 1.5 and 4 N/mm.Peel strength is a metric of solder joint strength for solar cells andis an indicator of module reliability.

Meier et al. describes how to use a four-point probe electricalmeasurement to determine the resistivity of each metallization layer ona completed solar cell (Reference: Meier et al. “Determining componentsof series resistance from measurements on a finished cell”, IEEE (2006)pp1315). The bulk resistance of a metallization layer is directlyrelated to the bulk resistance of the material from which it is made. Asan example, the bulk resistance of pure Ag is 1.6E-8 Ω-m. However,because of the structural imperfections and impurities, Ag metallizationlayers used on industrial solar cells can have a bulk resistance that is1.5 times to 5 times higher than the bulk resistance of pure Ag. A lowbulk resistance is especially important for fine grid lines, which musttransport current over a relatively long (i.e., more than 1 cm) length.

In a solar cell current will flow from an aluminum layer on the backside and through a rear tabbing (or metallization) layer. The contactresistance between the rear tabbing layer and the aluminum layer in thesolar cell can be measured by using the transmission line measurement(TLM) (Reference: Meier et al. “Cu Backside Busbar Tape: Eliminating Agand Enabling Full Al coverage in Crystalline Silicon Solar Cells andModules”, IEEE PVSC (2015) pp. 1-6). The TLM is plotted as resistanceversus distance between electrodes. In the experimental set-up the reartabbing layer is printed directly onto a silicon wafer and then dried.An aluminum layer is subsequently printed partially over the silverlayer with an overlap region of approximately 300 μm surrounding allsides of the tabbing layer. The wafer is subsequently dried andeventually co-fired. The contact resistance between the rear tabbinglayer and the aluminum layer is half of the y-intercept value of alinear fit of the resistance versus distance TLM plot. The electricalresistance between busbars can be measured using a Keithley 2410Sourcemeter in a four-point probe setup that sourced current between−0.5 A and +0.5 A and measured the voltage. In various embodiments, thecontact resistance between the rear tabbing layer and the aluminum layeris between 0 and 5 mΩ, 0.25 and 3 mΩ, 0.3 and 1 mΩ, or any rangesubsumed therein. The sheet resistance of the aluminum layer isdetermined by the slope of the line times the length of the electrodes.The contact resistance and sheet resistance can be used to numericallydetermine the transfer length and subsequently the contact resistivity.

The contact resistance between a metallization layer and an underlyingsilicon wafer has an impact on the power conversion efficiency of asolar cell. The contact resistance can be measured on silicon substratesby using the TLM described above. For fine grid lines on the front sideof a silicon wafer, it is useful to reduce the contact resistance toless than 30 mΩ-cm² or less than 10 mΩ-cm² to maintain a high fillfactor. For the rear tabbing and front busbar layers the contactresistance can be higher than for the fine grid lines and still notaffect device performance because most of the current is extracted fromthe wafer through the fine grid lines on the front side and the aluminumlayer on the rear side. Furthermore, there can be a large contactresistance between a silver metallization layer and a silicon substratebecause silver does not make ohmic contact to non-degenerately doped,p-type silicon. Furthermore, such a contact resistance tends to increasewhen a large quantity of base metal particles are substituted for silverparticles in the metallization paste. In some embodiments, the contactresistance between the stratified film and the silicon wafer is greaterthan 100 mΩ-cm², or greater than 1 Ω-cm², or greater than 10 Ω-cm².

Ag-LowT_(M)BM Metallization Paste Components

This disclosure describes pastes made of silver and low-melting-point,base metal particles (LowT_(M)BMs) metallization pastes (Ag-LowT_(M)BMpastes). The Ag-LowT_(M)BM pastes contain silver particles, LowT_(M)BMparticles, aluminum based particles, and an organic vehicle. Such pastesmay also contain crystallizing agent(s) and small amounts of strongglass-forming frits. The term “solids loading” is often used inconnection with metallization pastes to mean the amount or proportion ofinorganic solids in a metallization paste. In an exemplary embodiment,Ag-LowT_(M)BM metallization paste has a solids loading that includessilver particles, low-melting-point base-metal particles, aluminum-basedparticles, no crystallizing agent and no strong glass-forming frits. Allmetallization pastes described herein also include an organic vehicle,although that may not always be stated explicitly.

D50 is a common metric that is used to describe the median diameter ofparticles. The D50 value is defined as the median value at which half ofthe particle population has a diameter below and half the particlepopulation has a diameter above the value. Measuring a particle diameterdistribution is typically performed with a laser particle size analyzersuch as the Horiba LA-300. Spherical particles are dispersed in asolvent in which they are well separated and the scattering oftransmitted light is directly correlated to the size distribution fromsmallest to largest dimensions. A common approach to express laserdiffraction results is to report the D50 values based on volumedistributions. It should be understood that the term “spherical shape”is used herein to mean an approximately spherical or equiaxed shape.Particles do not generally have perfect spherical shapes. It is alsopossible to measure the D50 and particle size distribution of particlesusing a scanning electron microscope and image processing software suchas ImageJ.

In one arrangement, the silver particles in the metallization paste arespherically shaped, nanoparticles with a D50 between 10 nm and 1 μm, orbetween 50 nm and 800 nm, or between 200 nm and 500 nm, or any rangesubsumed therein. In another arrangement, the silver particles in themetallization paste are spherically shaped, micron-sized particle with aD50 between 1 μm and 10 μm, or between 1 μm and 5 μm, or between 1 μmand 2 μm, or any range subsumed therein. In another arrangement, silverparticles have flake, dendrite, or filament shapes. In an exemplaryembodiment, the silver particles are flakes with a width between 1 and10 μm and a thickness between 100 nm and 500 nm. A silver filament mayhave a diameter between 200 nm and 1000 nm and a length greater than 1μm. In an exemplary embodiment, the silver particles have a unimodalsize distribution. In another exemplary embodiment, the silver particleshave a bimodal particle size distribution. In general, silver particlescan include a mixture of spherical nanoparticles, spherical micron-sizedparticles, flakes, dendrites and/or filaments. In an exemplaryembodiment, the silver particles contain a mixture of nanoparticles andmicron sized particles. In one embodiment, the silver particles contain50 wt % spherical nanoparticles with a D50 of 500 nm, 30 wt % sphericalmicron sized particles with a D50 of 2 μm, and 20 wt % silver flakeswith a diameter of 2 μm.

The term “low-melting-point, base-metal” (LowT_(M)BM) particle is usedherein to describe any base-metal-containing or base-metal alloyparticle that has a melting point below 400° C., below 350° C., or below300° C. In an exemplary embodiment the LowT_(M)BMs have a solubility insilver of less than 20 wt % or less than 10 wt % at 850° C. In oneembodiment of the invention, LowT_(M)BMs contain bismuth (Bi), tin (Sn),tellurium (Te), antimony (Sb), lead (Pb), or alloys, composites, orother combinations thereof. In one arrangement, the LowT_(M)BMs areprimarily crystalline and metallic. In one arrangement, the LowT_(M)BMparticles have a spherical shape with a D50 between 50 nm and 5 μm,between 300 nm and 5 μm, or between 300 nm and 2 μm. In an exemplaryembodiment, the LowT_(M)BM particles have a unimodal size distribution.In an exemplary embodiment, the LowT_(M)BM particles have a bimodalparticle size distribution. In another arrangement, the LowT_(M)BMparticles have a flake, dendrite, or filament shape. The flake may havea diameter between 1 μm and 10 μm and a thickness between 100 nm and 500nm. The filament may have a diameter between 200 μm and 1000 nm and alength greater than 1 μm. In an exemplary embodiment the LowT_(M)BMparticles have a D50 of 2 μm.

In other embodiments, a low-melting-point, base-metal particle has acore-shell morphology; a LowT_(M)BM core particle is coated with a thinshell. In some arrangements, the coat completely encapsulates the coreparticle. In other arrangement, the coat only partially encapsulates thecore particle. In one embodiment, a core-shell LowT_(M)BM particle has aLowT_(M)BM core and a first shell coating the core. The first shell maycontain nickel, boron, silver, gold, platinum, copper, indium, tin,zinc, lead, bismuth, antimony, or alloys or combinations thereof. Thefirst shell may have a uniform thickness or the thickness of the shellmay vary. In various embodiments, on average, the first shell is lessthan 1000 nm thick, less than 500 nm thick, less than 200 nm thick, lessthan 50 nm thick or any range subsumed therein. In an exemplaryembodiment, the core-shell LowT_(M)BM particle has a metallic bismuthcore and a first silver shell that is 200 nm thick. In anotherembodiment, a core-shell LowT_(M)BM particle has a metallic bismuth coreand a nickel-boron alloy (2-8 wt % B) first shell.

Glass frits are glassy oxides of silicon or boron and one or moreadditional elements such as barium, bismuth, lead, zinc, tellurium,aluminum, strontium, sodium, lithium, or other trace heavy metals. Whenused in conventional silver metallization pastes, the composition of thefrit is chosen to provide optimal melting and flow properties forsintering of the silver metal particles at temperatures between 400° C.and 900° C. Commercially available glass frit (e.g., Ceradyne product#V2079) and other additives can be used in front side metallizationpastes to penetrate through anti-reflective coatings, improve silversintering, and make ohmic contact to the silicon wafer.

The reactivity of a glass frit during co-firing determines, at leastpartially, whether it will assist in crystallization or impedecrystallization. Strong glass-forming frits may contain, in oxide andalloy forms, bismuth (Bi), lead (Pb), silicon (Si), and sodium (Na).Such strong glass-forming frits can readily solubilize metals intoamorphous networks. Many glass frits commonly used in PV metallizationpastes are strong glass-forming frits. Strong glass-forming frits canincrease the oxidation of LowT_(M)BM making them much less effective inmetallization paste. A film formed from a metallization paste thatcontains strong glass-forming frits, silver particles, and LowT_(M)BMmay have poor solderability with solder-coated ribbons and may also havea brittle internal structure due to the large volume of glassy materialwhen co-fired under conditions typically used for aluminum back surfacefield (BSF) solar cells. In Ag-LowT_(M)BM metallization pastes, strongglass forming frits, such as those containing, in oxide and alloy forms,mainly bismuth (Bi), lead (Pb), silicon (Si), sodium (Na), and boron (B)are used sparingly in order to prevent oxidation of LowT_(M)BMs. In onearrangement, such a metallization paste contains less than 2 wt % strongglass-forming frits.

Weak glass-forming frits, which may contain, in oxide and alloy forms,tellurium (Te), silicon (Si), boron (B), aluminum (Al), or zinc (Zn).Such weak glass-forming frits solubilize metals into amorphous networksonly minimally. In some arrangements, films formed from metallizationpastes that contain such weak glass-forming frit material show partialformation of crystalline phases within a glassy phase. Such films havegood high temperature solderability and strong peel strengths.

The term “crystallizing agent” is used herein to describe a materialthat reacts with LowT_(M)BMs during co-firing to assist in formation ofcrystals in a LowT_(M)BM sublayer. Crystallizing agents may also improvethe adhesion and subsequent peel strength of co-fired Ag:LowT_(M)BMslayers. The crystallizing agent may be in the form of particles. In oneembodiment, crystallizing agents are a specific subgroup of glass fritsthat contain tellurium (Te), silicon (Si), boron (B), aluminum (Al),zinc (Zn), or alloys, oxides, composites, or other combinations thereof.It is common to add crystallizing agents to boron-oxide-containing orsilicon-oxide-containing glasses. In another embodiment, a crystallizingagent contains metals or metallic alloys of tellurium (Te), silicon(Si), boron (B), or zinc (Zn), composites, or other combinationsthereof. In an exemplary embodiment, a crystallizing agent containstellurium. In an exemplary embodiment, a crystallizing agent containstellurium oxide. In one arrangement, the crystallizing agent is aparticle with a spherical shape and a D50 between 50 nm and 5 μm,between 100 nm and 3 μm, between 200 nm and 1 μm, or any range subsumedtherein. In another arrangement, the crystallizing agent has a flake,dendrite, or filament shape. The flake may have a diameter between 1 and10 μm and a thickness between 100 nm and 500 nm. In one exemplaryembodiment, crystallizing agent particles have a unimodal sizedistribution. In another exemplary embodiment, crystallizing agentparticles have a bimodal particle size distribution.

In one embodiment of the invention, the organic vehicle is a mixture oforganic solvents and binders. Other additives may also be included inthe organic vehicle. The viscosity of metallization pastes can be tunedby adjusting the amounts of binders and solvents in the organic vehicleand by including thixotropic agents. In one arrangement, themetallization paste has a viscosity between 10,000 and 200,000 cP at 25°C. and at a sheer rate of 4 sec⁻¹ as measured using a temperaturecontrolled Brookfield DV-II Pro viscometer. Common solvents includeterpineol and glycol ethers (diethylene glycol monobutyl ether,triethylene glycol monobutyl ether, and texanol). Common organic bindersinclude ethyl cellulose, carboxymethyl cellulose, poly(vinyl alcohol),poly(vinyl butyral), and poly(vinyl pyrrolidinone).

Aluminum-based particles may be added to a Ag:Bi metallization paste inorder to reduce contact resistance between rear tabbing and aluminumlayers. In an exemplary embodiment, aluminum-based metal particles aremade of metallic aluminum (Al). In one embodiment aluminum-basedparticles are crystalline. In one arrangement, aluminum-based particleshave a spherical shape with a D50 between 50 nm and 15 μm, between 300nm and 10 μm, or between 300 nm and 4 μm. In an exemplary embodiment,aluminum-based particles have a unimodal size distribution. In anexemplary embodiment, aluminum-based particles have a bimodal particlesize distribution. In some arrangements, aluminum-based particles haveflake, dendrite, or filament shapes. The flake may have a diameterbetween 1 μm and 10 μm and a thickness between 100 nm and 500 nm. Thefilament may have a diameter between 200 μm and 1000 nm and a lengthgreater than 1 μm.

In one embodiment of the invention, LowT_(M)BM metallization pastes andfilms also contain additional materials that can getter oxygen orincrease their shelf lives (e.g., Staybelite™). In an exemplaryembodiment, semiconducting particles such as silicon are added to abismuth-containing metallization paste so that bismuth silicate crystalsare formed during co-firing in the LowT_(M)BM layer formed from such apaste, increasing the degree of crystallinity in the LowT_(M)BM layer.

In one embodiment of the invention, at least some silver particle in ametallization paste are replaced by LowT_(M)BMs. Such pastes can bereferred to as Ag:LowT_(M)BM pastes. Crystallizing agents can improvethe adhesion and peel strength of films made from co-firedAg:LowT_(M)BMs pastes.

Ag-LowT_(M)BM Paste Formulations for Specific Metallization Layers

The low-melting-point base-metal fraction is a useful metric that can beused to describe the LowT_(M)BM content in pastes and films. Thelow-melting-point base-metal fraction is determined by dividing theweight of the LowT_(M)BM by the entire metal weight in the metallizationpaste. As an example, if a paste contains 3.5 g of Ag and 2 g ofbismuth, which is the LowT_(M)BM, and 0.1 g of aluminum based particles,then the low-melting-point base-metal fraction in the paste is 35.7% andis calculated using the equation below:

${{Low}\; T_{m}{BP}\mspace{14mu} {Metal}\mspace{14mu} {Fraction}} = {{\frac{2\mspace{14mu} g\mspace{14mu} {Bi}}{{2\mspace{14mu} g\mspace{14mu} {Bi}} + {3.5\mspace{14mu} g\mspace{14mu} {Ag}} + {0.1\mspace{14mu} g\mspace{14mu} {Al}}}*100\%} = {35.7\%}}$

In exemplary embodiments, the metallization paste has alow-melting-point base-metal fraction greater than 20%, greater than30%, greater than 40%, or greater than 50%.

Examples of formulations for rear tabbing and front busbar pastes thatcontain silver particles, LowT_(M)BMs, aluminum based particles andcrystallizing agents are shown in Table I. In one embodiment of theinvention, a Ag-LowT_(M)BMs metallization paste has a solids loadingbetween 30 wt % and 70 wt %. In one embodiment, the metallization pastehas between 10 wt % and 45 wt % silver particles. In one embodiment, themetallization paste has between 5 wt % and 35 wt % LowT_(M)BM particles.A small quantity of aluminum-based particles can be included in thepaste to reduce contact resistance between a rear tabbing and analuminum layer. In another embodiment, the metallization paste hasbetween 0.5 wt % and 3 wt % aluminum based particles. Crystallizingagents are typically added when it is desirable to increase the peelstrength of the rear tabbing layer or front busbar layer. In variousembodiments, a crystallizing agent accounts for less than 1 wt %, lessthan 0.5 wt %, or less than 0.1 wt % of the paste. In one embodiment, ametallization paste also contains less than 2 wt %, less than 1 wt %, or0 wt % strong glass-forming frits.

TABLE I Silver/LowT_(M)BMs/Crystallization Agent Metallization PasteFormulations (wt %) Crystal- Aluminum- Paste Use Silver lizing basedOrganic (lines) particles LowT_(M)BMs Agent particles Vehicle RearTabbing 10-45 5-35 0.1-3 0 30-70 (I) Rear Tabbing 10-45 5-35  0-3 0.5-330-70 (II) Front Busbar 10-45 3-35 0.1-3  0-3 30-70

Metallization paste formulation can be adjusted to achieve a desiredbulk resistance, contact resistance, film thickness, and/or peelstrength for a particular metallization layer. In an exemplaryembodiment, the metallization paste contains 35 wt % Ag particles, 16 wt% metallic bismuth particles (LowT_(M)BMs), 1 wt % aluminum-basedparticles, no glass frit, no crystallizing agent and 48 wt % organicvehicle. In another embodiment, the metallization paste contains 35 wt %Ag particles, 16 wt % metallic bismuth particles (LowT_(M)BMs), 0.07 wt% TeO₂ (crystallizing agent), no glass frit, no aluminum-based particlesand 48.93 wt % organic vehicle.

Stratified Metallization Film Made From Ag-LowT_(M)BM Pastes

Scanning electron microscopy (SEM) and energy dispersive x-rayspectroscopy (EDX) (referred to collectively as SEM/EDX) as used hereinwere performed using a Zeiss Gemini Ultra-55 analytical field emissionscanning electron microscope, equipped with a Bruker) XFlash® 6|60detector. Details about operating conditions are described for eachanalysis. Cross-sectional SEM images of the co-fired multilayer filmstack were prepared by ion milling. Samples are prepared by applying athin epoxy layer to the top of the co-fired multilayer stack and driedfor at least 30 minutes. The sample was then transferred to a JEOLIB-03010CP ion mill operating at 5 kV and 120 uA for 8 hours to remove80 microns from the sample edge. Milled samples were stored in anitrogen box prior to SEM/MX.

FIG. 1 is a schematic cross-section drawing of a stratified film 100that was formed during co-firing a Ag: LowT_(M)BM paste on a siliconsubstrate 110. It should be noted that the figure is not drawn to scale.During co-firing, at least a portion of the silver particles andLowT_(M)BM particles in the Ag:LowT_(M)BM paste phase separate from eachother. The LowT_(M)BM particles may melt and collect, forming aLowT_(M)BM sublayer 120 adjacent to the silicon substrate 110. Thesilver particles may sinter or melt, forming a silver-rich sublayer 130over the LowT_(M)BM sublayer 120. An outside surface 130S of thesilver-rich sublayer 130 is exposed to the outside environment. In oneembodiment, the LowT_(M)BM sublayer 120 contains bismuth, aluminum,carbon, tin, tellurium, antimony, lead, silicon, or alloys, oxides,composites, or combinations thereof. In an exemplary embodiment, theLowT_(M)BM sublayer 120 contains bismuth and oxygen (oxidized bismuth).In one arrangement, elements such as oxygen, aluminum, silicon, and/orcarbon that are incorporated into the LowT_(M)BM sublayer 120 duringco-firing.

The silver-rich sublayer 130 contains mostly silver. The outermostsurface 130S of the silver-rich sublayer is easily soldered as it isalso silver-rich. Plan view EDX was used to determine the concentrationof elements on the outermost surface 130S of the silver-rich sublayer130. SEM/EDX was performed with the equipment described above and at anaccelerating voltage of 10 kV with a 7 mm sample working distance and500 times magnification. In various embodiments, at least 70 wt %, atleast 80 wt %, or at least 90 wt % of the outside surface 130S of themetallization layer is silver. An interface 140 between the LowT_(M)BMsublayer 120 and the silver-rich sublayer 130 may not be as sharplydefined as shown in FIG. 1. In some arrangements, there may be someintermixing of the LowT_(M)BM sublayer 120 and the silver-rich sublayer130 layers at the interface 140.

The thicknesses of the stratified film 100, the LowT_(M)BM sublayer 120and the silver-rich sublayer 130 can be measured using cross-sectionalSEM/EDX of the stratified film. In some embodiments, the total thicknessof the stratified film 100 varies between 1 μm and 15 μm, between 1 μmand 10 μm, between 1 μm and 3 μm, or any range subsumed therein. In someembodiments, the silver-rich sublayer 130 has a thickness between 0.5 μmand 10 μm, between 0.5 μm and 5 μm, between 1 μm and 4 μm, or any rangesubsumed therein. In some embodiments, the LowT_(M)BM sublayer 120 has athickness between 0.01 μm and 5 μm, between 0.25 μm and 5 μm, between0.5 μm and 2 μm, or any range subsumed therein.

The low-melting-point base-metal fraction in the stratified film 100 canbe determined by Energy Dispersive X-Ray Spectroscopy (EDX). EDX can beused to measure the ratio of Ag to Bi even if the bismuth is in acrystalline or oxidized state. The EDX data was collected forapproximately three minutes using the equipment described above at anaccelerating voltage of 20 kV, and a working distance of 7 mm over theentire image shown in FIG. 4. Elemental quantification was performed onthe spectra using Bruker Quantax Esprit 2.0 software for automaticelemental identification, background subtraction, and peak fitting. Onlythe metal peaks, which were exclusively silver and bismuth, quantifiedfrom the EDX spectrum to determine the low-melting-point base-metalfraction in the film. In an exemplary embodiment, the stratified filmhas a low temperature base-metal fraction greater than 20 wt %, orgreater than 30%, or greater than 40% or greater than 50%.

In the past, it was challenging to increase the low-melting-pointbase-metal fraction in a silver-based metallization paste beyond 16 wt %with materials such as bismuth because of Bi oxidation during heating.In one embodiment, the addition of the crystallizing agent and/or thealuminum-based particles reduces oxidation, allowing an increase in theLowT_(M)BMs metal fraction beyond 20 wt %. In another embodiment, thereduction of glass frits to less than 3 wt % can reduce oxidation of theLowT_(M)BM during co-firing, which improves solderability.

A crystallizing agent may help the LowT_(M)BM to form crystallinecompounds with silicon and oxygen during co-firing. Such crystallinecompounds can improve adhesion between the LowT_(M)BM sublayer 120 andthe silicon surface 110. In one embodiment of the invention, thestratified film 100 contains (MO_(x))_(y)(SiO₂)_(z), crystallites,wherein 0≤x≤3, 1≤y≤10, and 0≤z≤1. M may be any of bismuth (Bi), tin(Sn), tellurium (Te), antimony (Sb), lead (Pb), and mixtures thereof.Such crystalline compounds may be distributed throughout the film eitherhomogeneously or heterogeneously in a stratified manner and may accountfor 0.1-20 wt %, 0.5-10 wt %, 1-5 wt % (or any range subsumed therein)of the co-fired film. In one embodiment, the stratified film 100contains less than 1 wt % crystallizing agent. The crystallizing agentmay contain tellurium (Te), silicon (Si), boron (B), zinc (Zn), oroxides thereof. In an exemplary embodiment, the stratified layer 100contains less than 1 wt % or less than 0.1 wt % tellurium. Thecrystallites in the LowT_(M)BM sublayer may increase the internalstrength of the sublayer 120, acting as a support structure.Aluminum-based materials may also be included in Ag:LowT_(M)BM films toreduce contact resistance in overlap regions on the back side of a solarcell, which be described in more detail below. In an exemplaryembodiment, the stratified film contains at least 0.5 wt % aluminum.

The contact resistance between the LowT_(M)BM sublayer 120 and thesilicon substrate 110 is greater than 100 mΩ-cm², greater than 1 Ω-cm²,or greater than 10 Ω-cm².

In some embodiments, there is a passivation layer (not shown), such as asilicon nitride (Si₃N₄) layer between the silicon substrate 110 and thestratified film 100. Such a passivation layer lowers the surfacerecombination velocity at the interface between the silicon substrate110 and the stratified film 100. The stratified film 100 is in contactwith the passivation layer instead of directly contacting the surface ofthe silicon substrate 110. Pastes that do not etch through thedielectric layer is known as a “floating” metallization pastes. Floatingmetallization pastes can be made by careful selection of crystallizingagents and LowT_(M)BMs. The degree to which the paste etches through theSi₃N₄ can be measured by first co-firing the paste on a Si₃N₄ coatedsilicon wafer followed by etching the silver film in an appropriatestrong acid based etchant and measuring the area removed under themetallization layer using an optical microscope. Silicon nitridesections that are not etched remain blue while etched regions showexposed silicon. When more than 90% of the metallized area contain theSi₃N₄ after etching, a a paste can be considered to be a floatingmetallization paste. For floating busbar pastes it is advantageous touse non-fire through pastes so that penetration of the passivation layerdoes not occur. In other exemplary embodiments, the Ag:LowT_(M)BMmetallization paste is a floating paste.

Solar Cell Fabrication Using Ag:LowT_(M)BM Based Rear Tabbing Pastes

FIG. 2 is a schematic drawing that shows the front (or illuminated) sideof a silicon solar cell 200. The silicon solar cell 200 has a siliconwafer 210 with an anti-reflective coating (not shown), and in somecases, a dielectric layer (not shown) on the back side. In variousembodiments, the silicon wafer is monocrystalline, or multi-crystalline,and has an n-type, or a p-type base. The solar cell has fine grid lines220 and front side bus bars lines 230. In one embodiment, Ag:LowT_(M)BMmetallization pastes are used as drop-in replacements for commercialAg-based pastes to form the front side bus bars lines 230. First, acommercially-available front-side metallization paste is screen printedand dried at 150° C. to form the fine grid lines 220. Then, theAg:LowT_(M)BM metallization paste is screen printed and dried at 150° C.to form the front busbar layers 230. In one embodiment, the frontbusbars do not penetrate through the anti-reflective coating. The reartabbing paste and rear aluminum paste are subsequently printed anddried, and the entire wafer is co-fired to over 750° C. forapproximately one second in air. In one arrangement, screen-printedfront busbar layers 230 range in thickness from about 2 to 15 μm afterco-firing. The front busbar layers 230 have a bulk conductivity that is2-50 times, or 2-25 times, or 2-10 times less than the bulk conductivityof silver, which has a bulk resistivity of about 1.6E-8 Ω-m.

FIG. 3 is a schematic drawing that shows the rear (or back) side of asilicon solar cell 300. Some regions of the rear side of the solar cellare coated with an aluminum layer 330 and some regions are coated withhas rear side tabbing layers 340 distributed over a silicon wafer 310.In some embodiments, the aluminum layer 330 extends over a portion ofthe rear tabbing layers 340 creating overlap regions 350. In oneembodiment, Ag:LowT_(M)BM metallization pastes are used as drop-inreplacements for commercial Ag-based metallization pastes to form therear tabbing layers 340. The Ag:LowT_(M)BM rear tabbing paste is screenprinted onto portions of the silicon wafer 310 and dried at 150° C. toform the rear tabbing layer 340. An aluminum paste is subsequentlyprinted onto the remaining portions of silicon substrate 310 that havenot already been printed with the Ag:LowT_(M)BM rear tabbing paste toform the aluminum layer 330. A portion of the aluminum layer 330overlaps a portion of the rear tabbing layers 340 in the overlap regions350. The solar wafer is subsequently dried at 250° C. for 30 seconds to10 minutes. Prior to co-firing, the overlap region 350 contain a layerof dried Ag:LowT_(M)BM paste on the silicon surface and a dried aluminumlayer over the layer of dried Ag:LowT_(M)BM paste. Acommercially-available front side metallization paste is screen printedand dried at 150° C. to form the fine grid lines 220 and busbar 230layers (FIG. 2). Then the silicon solar cell is co-fired at over 750° C.for approximately one second, which is a common temperature profile foraluminum BSF silicon solar cells. During co-firing it is common for theregions of the aluminum layer 330 that are in direct contact with thesilicon wafer to form both a back surface field and a solidifiedaluminum-silicon eutectic layer. After co-firing the overlap region 350contains a solid aluminum-silicon eutectic layer on the silicon wafer310, a modified rear tabbing layer on the solid aluminum-siliconeutectic layer and an aluminum layer over the modified rear tabbinglayer. In one arrangement, screen-printed rear tabbing layers range inthickness between about 1.5 and 7 μm after co-firing. The rear tabbinglayer has a bulk conductivity that is 2-50 times, 2-25 tines, or 2-10times less conductive than bulk silver, which has a bulk resistivity ofabout 1.6E-8 Ω-m.

In another embodiment, the rear surface of the silicon substrate has adielectric layer, and the rear tabbing layers do not penetrate throughthe dielectric layer. In another embodiment, the rear surface of thesilicon substrate has a doped silicon base layer, and the rear tabbinglayers make electrical contact to the silicon base layer with a contactresistance greater than 100 mΩ-cm².

In some embodiments silicon solar cells are connected to one another bysoldering tin-coated copper tabbing ribbons to the front busbars andrear tabbing layers. Solder fluxes that are commercially designated aseither RMA (e.g., Kester® 186) or R (e.g., Kester® 952) are deposited onthe tabbing ribbon and dried at 70° C. A tinned copper ribbon, which isbetween 0.8 and 3 mm wide and 100-300 um thick, is then placed on thesolar cell and contacted to the front busbars and the rear tabbinglayers with a solder iron at a temperature between 200° and 400° C. Thesolder joints formed during this process have a mean peel strength thatis greater than 1 N/mm (e.g., a 2 mm tabbing ribbon would require a peelforce of greater than 2N to dislodge the tabbing ribbon).

Silver Paste Containing LowT_(M)BMs and Crystallizing Agent to ImprovePeel Strength

In the past, routine optimization of conventional glass fritconcentrations could not achieve optimal morphologies in Ag-LowT_(M)BMmetallization pastes. Adjusting the proportion of conventional fritsthat are used in solar metallization pastes within the ranges that havebeen used in the past (i.e., 3-8 wt %) produced only weak films even atthe lowest frit loadings. The most commonly used frits are bismuth-basedor lead-based, which can be strongly glass-forming and can causeamorphization of the LowT_(M)BM layer. Because the glassy phase formedfrom frit has typically been considered to be a source of internalstrength and adhesion to the silicon substrate, it would have beencounterintuitive to attempt to crystallize the glassy phase of acomposite film formed under specific ratios of Ag, Bi, and weakglass-forming frits, especially in the absence of the crystallinephases.

Table II below shows the compositions of several pastes that were screenprinted as busbars and tabbing layers on a silicon substrate, dried, andco-fired over a range of firing conditions typically used forconventional Al BSF (aluminum back surface field), multicrystallinesolar cells. Each substrate with paste was placed on a mat, heated to65° C., and soldered with a tip temperature of 285° C. using a copperribbon with Sn₆₀Pb₄₀ solder that was fluxed with Alpha® NR-205 and driedat 70° C. The metal lines were subsequently peeled at 180° over adistance greater than 60 mm. Peel strengths averaged over severalco-firing conditions for each paste are also shown in Table I. The datashows clearly the importance of including a crystallizing agent andminimizing strong glass-forming frits in the Ag:LowT_(M)BMs system inorder to maximize peel strength.

TABLE II Silver/LowT_(M)BMs/Crystallizing Agent Metallization PasteFormulations (wt %) Average Silver LowT_(M)BM Crystallizing StrongGlass- Organic Peel Paste particles particles Agent Forming Frit VehicleStrength Paste A 35 20 0 0 45 2.4 N/mm Paste B 35 20 1 0 44 4.3 N/mmPaste C 35 20 0 1 44 1.8 N/mm

Paste A is a control paste that contains 35 wt % Ag particles, 20 wt %metallic Bi particles as the LowT_(M)BM particles, and no crystallizingagent or glass frit. Paste A has an average peel strength of 2.4 N/mm.Peel strength for films made from Paste A drop below 2 N/mm whenco-fired at lower temperatures typically used for passivated emitterrear contact (PERC) solar cell architectures.

Paste B contains 35 wt % Ag particles, 20 wt % of metallic Bi particlesas the LowT_(M)BMs, 1 wt % tellurium oxide frit, which acts as thecrystallizing agent, and no strong glass-forming frits (i.e., no Bi₂O₃or PbO containing frit), according to an embodiment of the invention.Adding only 1 wt % crystallizing agent increases the average peelstrength of the film to 4.3 N/mm, and these values are consistently highover a range of firing temperatures greater than 100° C. The viscosityof Paste B is approximately 80,000 cP at 25° C. and at a sheer rate of 4sec⁻¹.

Traditionally, glass frits used in Ag metallization pastes have beenmade from Bi and Pb oxide derivatives, which are strong glass-formingfrits. These strong glass-forming frits do not make strong films whenlarge concentrations of LowT_(M)BMs such as Bi are also included in thepaste. Traditional frits can aid in the oxidation of Bi and result infilms that have very poor solderability. Paste C, shown in Table I,contains 35 wt % Ag particles, 20 wt % of metallic Bi particles as theLowT_(M)BMs and 1 wt % of Bi₂O₃ containing frit. The average peelstrength for films made from Paste C is 1.8 N/mm, which is significantlylower than the peel strength of Paste B which contains the crystallizingagent.

FIG. 4 is a scanning electron microscope (SEM) image that shows apolished cross section of a stratified metal film 400 on a siliconsubstrate 410 made from Paste B. The image shows a LowT_(M)BM sublayer420 adjacent to the silicon substrate 410. The LowT_(M)BM sublayer 420is approximately 1 μm-thick and contains mainly bismuth, oxygen andsilicon. The bright regions in the LowT_(M)BM sublayer 420 arecrystallites 440 that have been identified as bismuth silicates. Thereis a silver-rich sublayer 430 that is approximately 1-2 μm-thick overthe LowT_(M)BM sublayer 420. Many of the crystallites 440 extend fromthe silicon substrate 410 into the silver-rich sublayer 430. In oneembodiment, the normalized weight ratio of silver to bismuth (Ag:Bi) inthe stratified metal film 400 is 1.8 to 1, resulting in alow-melting-point base-metal fraction of 35.7 wt % (1/(1.8+1)). Thecrystallizing agent in Paste B, which contains tellurium, representsless than 1 wt % of the stratified film.

X-ray diffraction was performed using a Bruker ZXS D8 Discover GADDSx-ray diffractometer equipped with a VÅNTEC-500 area detector and acobalt x-ray source operated at 35 kV and 40 mA. Diffractograms weremeasured using the cobalt Kα wavelength in two 25° frames which werecombined for a total window of 25-60° in 2Θ. Each frame was measured for30 minutes under x-ray irradiation. Background subtraction was performedon diffraction patterns. FIG. 5 is an x-ray diffraction (XRD) pattern ofthe stratified Ag:Bi film made from Paste B and shown in FIG. 4. Thesmall peaks in the pattern have been identified as crystalline bismuthsilicates. There are two families of crystallites in the stratifiedAg:Bi film made from Paste B and shown in FIG. 4. They have beenidentified using XRD and EDX. Crystallites formed by the interaction ofthe bismuth and silicon in the presence of the crystallizing agent werefound to be (Bi₂O₃)_(x)(SiO₂) (1≤x≤6); exemplary crystallites containedthe crystalline phases (Bi₂O₃)(SiO₂), (Bi₂O₃)₆(SiO₂), and (Bi₂O₃).

In one embodiment, the silver particles in the paste alloy with thebismuth silicate material to form crystalline and glassy compounds suchas Ag_(y)(Bi₂O₃)_(x)(SiO₂)_(1−y−x) (0.1≤x≤0.9, 0≤y≤0.9), which can beseen with EDX on metallization layers after they have been soldered andpeeled.

The bulk resistivity of the Ag/LowT_(M)BM film resulting from Paste Bwas determined to be 6E-6 ohm-cm, measured using the four-point probemeasurement. The bulk resistivity of Ag/LowT_(M)BM films is higher thanthe bulk resistivity of pure Ag (1.6E-6 ohm-cm), which is to be expectedas roughly one third of the Ag/LowT_(M)BM film contains a LowT_(M)BMslayer that is less conductive than Ag. However, the bulk resistivity ofthe Ag/LowT_(M)BM film is close to the values of rear tabbing layersmeasured on commercially available PV cells. Thus, even though the bulkresistivity of such novel films is less than for silver-only films, thenovel films have low enough resistivity to make them useful for varioussolar cell metallization applications.

With reference to FIG. 1, although the LowT_(M)BM layer 120 along thesilicon substrate 100 does not greatly impede the flow of currentparallel to the surface of the silicon substrate, the LowT_(M)BM layermay decrease the flow of current from the silicon substrate 110 to theAg sublayer 130. Such a stratified Ag:LowT_(M)BM layer on the front sideof a solar cell (i.e., n-type emitter layer/silicon nitride) has acontact resistance of more than 10 Ω-cm² when measured using TLM. Itshould be noted that the contact resistance of commercially availablefront side pastes are at least two orders of magnitude lower whenmeasured under the same conditions. Tellurium oxide based frits havepreviously been used in fine grid line pastes to reduce contactresistance. However, when LowT_(M)BM represents more than 20 wt % versusthe silver, the tellurium oxide based frit does not greatly improve thecontact resistance. The advantage of using tellurium oxide basedcrystallizing agent in our system may not be related to improving Agcrystallite formation at the Si interface. Rather the tellurium oxidebased crystallizing agent can, work to crystallize the LowT_(M)BMs layerbetween the Ag metallization layer 130 and silicon substrate 110,further separating the Ag from the silicon and may even impede Agcrystallite formation at the silicon interface, which is necessary forlow contact resistance.

Silver Paste Containing Aluminum-Based Particles to Reduce ContactResistance Between the Metallization Layer and the Aluminum Layer

One of the greatest challenges in increasing the amount of LowT_(M)BMsin metallization pastes is the possibility of high contact resistancebetween a rear tabbing layer and an aluminum layer. FIG. 6A is across-section schematic illustration that shows a portion of the rearside of a silicon solar cell before co-firing, according to anembodiment of the invention. FIG. 6A includes a rear tabbing region 660,an overlap region 670, and an aluminum layer region 680, as indicated bydashed lines. In the rear tabbing region 660 and the overlap region 670a silicon substrate 610 is coated with a Ag:LowT_(M)BM rear tabbingpaste layer 625. In the aluminum layer region 680, the silicon substrate610 is coated with an aluminum layer 645. In one arrangement, thestructures in the overlap region 670 are created by printing the reartabbing paste layer 625 directly onto the silicon wafer 610, drying therear tabbing paste layer 625, and then printing the aluminum layer 645over the rear tabbing paste layer 625. In some embodiments, the overlapregion has a width between 10 μm and 500 μm, between 100 μm and 300 μmwide, or any range subsumed therein. In another embodiment of theinvention (not shown), the structures in the overlap region 670 arecreated in reverse order by printing the aluminum layer 645 directlyonto the silicon wafer 610, drying the aluminum layer 645, and thenprinting the rear tabbing paste layer 625 over the aluminum layer 645.

FIG. 6B is a cross-section schematic illustration that shows a portionof the rear side of the silicon solar cell shown in FIG. 6A afterco-firing. FIG. 6B includes the same rear tabbing region 660, overlapregion 670, and aluminum layer region 680 that are shown in FIG. 6A.During co-firing, various reactions occur. In the rear tabbing region660 the exposed portion 627 of the Ag:LowT_(M)BM rear tabbing pastelayer 625 has reacted to form a stratified layer 600 that includes anAg-rich sublayer and a LowT_(M)BM sublayer, as has been described abovein reference to FIG. 1.

In the aluminum layer region 680 the aluminum film 640 interacts withthe silicon wafer 610 (at temperatures above 660° C.) to form a solidaluminum-silicon eutectic layer 650. In one arrangement, there is asolid aluminum-silicon eutectic layer 650 in both the aluminum layerregion 680 and the overlap region 670, as shown in FIG. 6B. In onearrangement, the solid aluminum silicon layer 650 is continuous. Inanother arrangement, the solid aluminum silicon layer 650 is notcontinuous. Aluminum is a p-type dopant in silicon and, during firing,it can also form a highly p-type doped region that is known as the backsurface field (not shown).

In the overlap region 670 aluminum from the aluminum layer 640 andsilver from the covered portion 623 of the rear tabbing paste layer 625have interdiffused, forming a modified rear tabbing layer 630. The solidaluminum-silicon eutectic layer 650 may or may not form in the overlapregion 670. For a rear tabbing paste layer 625 that contains more than90 wt % silver, the Ag—Al interdiffusion can result in the formation ofa solid aluminum-silicon eutectic layer 650 in the overlap region 670.Such a solid aluminum-silicon eutectic layer in the overlap region 670may play a key role in reducing contact resistance between the modifiedtabbing layer 630 and the aluminum layer 640. For example, pure Ag reartabbing pastes with less than 10 wt % glass frit typically form layersthat have a contact resistance less than 3 mΩ between the rear tabbingand aluminum layers. For a rear tabbing paste layer 625 in which asignificant portion of silver particles have been replaced byLowT_(M)BMs, very little Ag—Al interdiffusion may occur and there may belittle or no formation of a solid aluminum-silicon eutectic layer in theoverlap region 670. High loadings of LowT_(M)BMs in rear tabbing pastesmay form layers that have extremely high contact resistance (i.e., morethan 10 mΩ) between the rear tabbing and aluminum layers. Duringco-firing most of the silver may interdiffuse into the aluminum layer640 leaving a modified rear tabbing layer 630 made of porous LowT_(M)BMmaterial and little or no solid aluminum-silicon eutectic layer 650.Silicon solar cells that have overlap regions 670 that contain porousmodified tabbing layers 630 and no solid aluminum-silicon eutectic mayhave high overall series resistance.

Aluminum and silver may interdiffuse during co-firing. In onearrangement, such interdiffusion result in a silver-aluminum region (notshown) that can extend by as much as 100 μm from the aluminum layer 640into the rear tabbing region 660.

In one embodiment of the invention, the backside of a solar cell has anoverlapping region that contains an aluminum layer and a rear tabbinglayer made from a Ag:LowT_(M)BM paste that does not containaluminum-based particles. The low-melting-point base-metal (bismuth)fraction in the paste is greater than 20 wt %. A polished, cross-sectionsample of the overlapping region was prepared for SEM imaging. An InLensmode scanning electron micrograph of the sample is shown FIG. 7. Therear tabbing region 760, the overlap region 770, and the siliconsubstrate 710 are indicated. The aluminum layer 740 is about 15 μm thickand the LowT_(M)BM-containing rear tabbing layer 720 is about 2.5 μmthick. The overlap region 770 contains a relatively porous modified reartabbing layer 730 and does not have a solid Al—Si eutectic layer. Thecontact resistance between the rear tabbing and aluminum layer for thissample is greater than 10 mΩ. overlap region.

Adding aluminum-based particles to the Ag:LowT_(M)BM stratified film canreduce the contact resistance between the silver-based rear tabbinglayer and the aluminum. In an exemplary embodiment, the aluminum-basedparticles in the Ag:LowT_(M)BM film assist in the formation of thealuminum-silicon eutectic formation during co-firing resulting in asolid Al—Si eutectic layer.

In another embodiment of the invention, the backside of a solar cell hasan overlapping region that contains an aluminum layer and a rear tabbinglayer made from a Ag:LowT_(M)BM paste that contains aluminum-basedparticles. The low-melting-point base-metal (bismuth) fraction in thepaste is greater than 20 wt %. A polished, cross-section sample of theoverlapping region was prepared for SEM imaging. An InLens mode scanningelectron micrograph of the overlap region 870 is shown FIG. 8. The reartabbing region 860, the overlap region 870, and the silicon substrate810 are indicated. The aluminum layer 840 is about 20 μm thick and theLowT_(M)BM-containing rear tabbing layer 820 is about 2 μm thick. Theoverlap region 870 contains a modified rear tabbing layer 830 thatappears to be non-porous, as well as a solid Al—Si eutectic layer 850.The contact resistance between the rear tabbing and aluminum layer forthis sample is less than 1.5 mΩ, which is similar to the contactresistance for silver rear tabbing pastes that contain no LowT_(M)BM. Invarious embodiments, the modified rear tabbing layer 830 contains atleast 1 wt % Al, at least 2 wt %, or at least 4 wt % aluminum. Invarious embodiments, the rear tabbing layer 820 contains at least 1 wt %Al, at least 2 wt %, or at least 4 wt % aluminum.

Other PV Cell Architectures

Ag:LowT_(M)BM based pastes can be used in other Si PV architectures suchas metal wrap through as well as passivated emitter rear contact (PERC)solar cells. The Ag:LowT_(M)BM based paste, described herein, can beused as a drop-in replacement for any metallization layers where a lowcontact resistance to silicon is not required. For the PERC architecturethe LowT_(M)BMs and crystallizing agents can be modified to initiatecrystallization at lower co-firing temperatures. In some instances,strong forming-glass frits may be used to initiate crystallization atlow firing temperatures.

This invention has been described herein in considerable detail toprovide those skilled in the art with information relevant to apply thenovel principles and to construct and use such specialized components asare required. However, it is to be understood that the invention can becarried out by different equipment, materials and devices, and thatvarious modifications, both as to the equipment and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

1. A solar cell comprising: a silicon substrate having a front surface and a back surface; a plurality of fine grid lines on the front surface of the silicon substrate; at least one front busbar layer on the front surface of the silicon substrate, the front busbar layer in electrical contact with at least a portion of the plurality of fine grid lines; an aluminum layer on a portion of the back surface of the silicon substrate; and at least one rear tabbing layer on a portion of the back surface of the silicon substrate; wherein the rear tabbing layer comprises a stratified film that comprises at least two sublayers: a first sublayer over the silicon substrate, the first sublayer comprising a low-melting-point base-metal; and a second sublayer over the first sublayer, the second sublayer comprising silver; wherein the second sublayer has an exterior surface, and the exterior surface is exposed to an outside environment; wherein the stratified film contains a low temperature base-metal fraction greater than 20%. wherein edges of the aluminum layer and edges of the rear tabbing overlap to form overlap regions that comprise: a solid aluminum-silicon eutectic layer on the silicon layer; a modified rear tabbing layer over the solid aluminum-silicon eutectic layer; and a portion of the aluminum layer over the modified rear tabbing layer.
 2. The solar cell of claim 1, wherein the stratified film further comprises (MO_(x))_(y)(SiO₂)_(z), crystallites wherein 0≤x≤3, 1≤y≤10, and 0≤z≤1 and M is a material selected from the group consisting of bismuth, tin, tellurium, antimony, lead, silicon, and alloys, composites, and combinations thereof.
 3. The solar cell of claim 1, wherein the first sublayer comprises a material selected from the group consisting of bismuth, aluminum, carbon, tin, tellurium, antimony, lead, silicon, and alloys, oxides, composites, and combinations thereof.
 4. The solar cell of claim 1, wherein the stratified film comprises at least 0.5 wt % aluminum.
 5. The solar cell of claim 1, wherein the stratified film comprises at least 1 wt % tellurium.
 6. The solar cell of claim 1, wherein the stratified film has a thickness between 1 μm and 15 μm.
 7. The solar cell of claim 1, wherein the first sublayer has a thickness between 0.01 μm and 5 μm.
 8. The solar cell of claim 1, wherein the second sublayer has a thickness between 0.5 μm and 10 μm.
 9. The solar cell of claim 1, wherein the second sublayer exterior surface comprises at least 70 wt % silver.
 10. The solar cell of claim 1, wherein the portion of the aluminum layer in the overlap region has an exterior surface, and the exterior surface is exposed to an outside environment.
 11. The solar cell of claim 1, wherein the overlap region is between 10 μm and 500 μm wide.
 12. The solar cell of claim 1, wherein the rear tabbing layers have a peel strength of more than 1 N/mm when soldered to tin-coated copper tabbing ribbons.
 13. The solar cell of claim 1, wherein a contact resistance between the rear tabbing layer and the aluminum layer is between 0 and 5 mΩ.
 14. A solar cell comprising: a silicon substrate having a front surface and a back surface; a plurality of fine grid lines on the front surface of the silicon substrate; at least one front busbar layer on the front surface of the silicon substrate, the front busbar layer in electrical contact with at least a portion of the plurality of fine grid lines; an aluminum layer on a portion of the back surface of the silicon substrate; and at least one rear tabbing layer on a portion of the back surface of the silicon substrate; wherein at least one of the front busbar layers or the rear tabbing layer comprises a stratified film that comprises at least two sublayers: a first sublayer over the silicon substrate, the first sublayer comprising a low-melting-point base-metal; and a second sublayer over the first sublayer, the second sublayer comprising silver; wherein the second sublayer has an exterior surface, and the exterior surface is exposed to an outside environment; wherein the stratified film contains a low temperature base-metal fraction greater than 20% 